This invention relates to an electrolyte-cathode assembly for a solid electrolyte fuel cell for operation at high temperature, comprising a solid ceramic electrolyte gas-tight body which is capable of conducting oxygen ions and at least one layer of an electron-conducting material in contact with the electrolyte and secured thereto.
It is known that in a certain kind of solid-electrolyte fuel cell which operates at a temperature usually of the order of 600.degree. to 1000.degree. C. the electrolyte is an oxide having cavities in its structure, such as stabilized zirconium dioxide, which has a relatively high oxygen ion conductivity in the aforementioned range of operating temperatures.
In a fuel cell of this kind the cathode is usually a layer of a metal, such as silver, which is a good conductor of electricity and does not oxidize under the cell operating conditions, the layer being in contact with the electrolyte and secured thereto.
Known metal cathodes are not entirely satisfactory, since they have the disadvantage of becoming detached, at least partly, from the electrolyte surface after a varying period of use, thus reducing their active surface. In addition, recrystallization may occur in metal cathodes and modify their porous structure and unfavorably influence their electrochemical properties and their lateral electric conductivity.
A considerable research effort has been made in order to find and develop cathodes having low polarization combined with a long life.
Swiss Patent Specification No. 478 463 describes an electrode which can be used both as a cathode and as an anode of a solid-electrolyte fuel cell operating at a high temperature. The electrode comprises a first porous layer made of granules of a ceramic material conducting oxygen ions and electrons, the first layer being in contact with the electrolyte and secured thereto, and a second porous layer covering the first layer and made of a material having high electron conductivity, such as a metal or ceramic material.
Swiss Patent Specification No. 540 571 describes an electrolyte-electrode assembly comprising a sealed-tight solid electrolyte body which is capable of conducting oxygen ions at high temperature and an electrode comprising at least one layer of doped indium oxide covering part of the surface of the electrolyte and adhering thereto, the doped indium oxide layer being made up of dendrites, the major axes of which are perpendicular to the surface of the electrolyte.
An object of this invention as exemplified hereinafter is to provide an electrolyte-cathode assembly in which the cathode has polarization characteristics at least as good as those of the electrodes described in the aforementioned Swiss Patent Specifications, but having a cathode life which is even better than that of the aforementioned electrodes.